Preferential Intergranular Oxidation and Stress Corrosion Cracking of Alloy X-750 in CO-CO2 Gas Mixtures
Bibliographic record
Abstract
The mechanical integrity of garter spring spacers used in CANada Deuterium Uranium (CANDU®) reactors are important to the integrity of pressure tubes that house fuel bundles. Brittle intergranular fracture was observed on Alloy X-750 ex-service spacers, instigating materials research to identify the responsible mechanisms. Alloy X-750 is a precipitation-hardened variant of Alloy 600 (72Ni-16Cr-9Fe) with the addition of Al and Ti, resulting in the formation of ’ precipitates, providing high strength. Alloy 600 and X-750 are susceptible to primary water stress corrosion cracking (PWSCC) in both pressurized water reactors (PWR) and boiling water reactors (BWR). Currently, preferential intergranular oxidation (PIO) is the most accepted mechanism for PWSCC, where the primary water environment (~300 °C hydrogenated water) enables selective oxidation of alloying elements, notably Cr, while Ni remains metallic. While spacers normally reside in a dry oxidizing mixture of CO2 and O2 within the annular gas system (AGS), extreme off-chemistry reducing conditions are being considered, which could promote PIO and embrittlement. In this project, PIO of Alloy X-750 was studied in 480 °C CO-CO2 reducing environments, in scenarios well away from the AGS but useful for establishing boundary conditions. Aged and annealed samples of Alloy X-750 were exposed to reducing CO-CO2 mixtures with the oxygen partial pressure ranging from 500× to 100,000× below the NiO oxygen dissociation pressure. Internal and intergranular oxidation were observed under all conditions, similar to Alloy 600 when exposed to 480 °C H2-steam in reducing thermodynamic conditions. Further nanoscale analysis of the samples tested at 500× and 5000× revealed that Al and Ti oxides lead PIO, while intergranular carbides hindered intergranular oxidation. Pre-notched samples tested in the 500× condition at 480 °C and with a stress intensity of 29.1 MPa×m0.5 resulted in fracture with characteristics analogous to PWSCC and internal oxidation: intergranular cracking driven by PIO with Ni nodules covering the fracture surface. In conclusion, Alloy X-750 spacers can undergo PIO in CO-CO2 gaseous environments via a PWSCC-like mechanism; however, this is conditional on the absence of O2 and maintaining a reducing environment for an extended period of time, both of which are unlikely in the AGS.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".